The invention relates to a filter for cleaning lubricating oil, especially for internal combustion engines of motor vehicles, having a filter case disposed in an at least approximately upright position when installed, and having also a cylinder-shaped oil filter cartridge inserted therein. The filter has also an oil inlet for the contaminated oil flowing around the cartridge and an oil outlet provided in a base at the bottom end of the filter case for filtered oil, this clean oil outlet being connected to the internal central part of the filter case. The filter also has a drain at the bottom end of the filter case; the drain can be connected to the inlet chamber of the filter case by removing the oil filter cartridge, and has a closure which shuts the drain off from the filter case.
An oil filter of the kind just described is disclosed in DE-OS No. 35 38 589. In this filter the closure is an annular, elastic sealing lip disposed externally on the bottom end of the filter cartridge when the latter is in the installed state; the sealing lip sealingly engages the inside wall of the filter case and is pointed in the normal oil flow direction in the manner of a circumferential check valve. When the filter cartridge is removed from the case the sealing lip arrives at an especially shaped area of the case wall, thereby opening the drain for the purpose of emptying the filter case before the cartridge is completely removed from the case. This oil filter does not have any functional disadvantage, but in practical application it has been found that the sealing lip represents a relatively large percentage of the cost of the filter, or, more precisely, of the filter cartridge. This is because the cost of manufacture is substantially higher in comparison to a simple sealing ring, and a high-quality material has to be used in order to assure the necessary resistance to heat. Furthermore, the sealing lip has to be provided new for each cartridge since both the lip and the cartridge are discarded after use.
The problem therefore presents itself of creating an oil filter of the kind described above, which will not have the disadvantage described, and which can be made at much lower cost.